Need reconfirmation on how tenacity stacking works

From what I understand, its currently possible 75% passive tenacity through Mercury's Treads, a fully stacked Legend: Tenacity from the Precision tree, and Unflinching from the Resolve tree if both summoners are on cooldown (80% if you just recently used a summoner and the other is on cooldown). However, I've recently started questioning exactly what that means. I THOUGHT this meant that crowd control would only last 25% of its normal duration on you (For instance Morgana's bind should be 0.75 seconds), but it doesn't really feel like that. Does this mean each instance of tenacity has diminishing returns and that's why these masteries aren't played very often or am I just being paranoid?